Setup linux server to use Exchange server as relay - linux

Question

It used to be working, but out of the blue, it stopped sending mail. I thought I set everything up in both linux and exchange to function correctly, but we're not receiving the emails - internally or externally.
I'm not that knowledgeable about linux, so I use webmin to get around.
So, we have our shopping cart and online forms on the linux server that will email users confirmations and such. It won't receive any mail, so I don't need to worry about that. It only sends mail out, both inside our network, and outside. Here is a recent addition to the mail log:
Dec 6 11:51:04 istalinux2 sendmail[1696]: rB6Gp4lr001696: from=www-data, size=246, class=0, nrcpts=1, msgid=, relay=www-data#localhost
Dec 6 11:51:05 istalinux2 sm-mta[1697]: rB6Gp4hY001697: from=, size=485, class=0, nrcpts=1, msgid=, proto=ESMTP, daemon=MTA-v4, relay=localhost [127.0.0.1]
Dec 6 11:51:05 istalinux2 sendmail[1696]: rB6Gp4lr001696: to="John Smith" jsmith#ista-in.org, ctladdr=www-data (33/33), delay=00:00:01, xdelay=00:00:01, mailer=relay, pri=30246, relay=[127.0.0.1] [127.0.0.1], dsn=2.0.0, stat=Sent (rB6Gp4hY001697 Message accepted for delivery)
Dec 6 11:51:05 istalinux2 sm-mta[1699]: rB6Gp4hY001697: to=jsmith#ista-in.org, delay=00:00:01, xdelay=00:00:00, mailer=relay, pri=120485, relay=10.20.51.30, dsn=5.1.2, stat=Host unknown (Name server: 10.20.51.30: host not found)
Dec 6 11:51:05 istalinux2 sm-mta[1699]: rB6Gp4hY001697: to=www-data#istalinux2.ista-in.org, delay=00:00:01, mailer=local, pri=120485, dsn=5.1.1, stat=User unknown
Dec 6 11:51:05 istalinux2 sm-mta[1699]: rB6Gp4hY001697: rB6Gp5hY001699: postmaster notify: User unknown
Dec 6 11:51:05 istalinux2 sm-mta[1699]: rB6Gp5hY001699: to=root, delay=00:00:00, xdelay=00:00:00, mailer=local, pri=30000, dsn=2.0.0, stat=Sent
I have a receive connector set up in Exchange to receive incoming mail from our linux server and pass it through.
I am using sendmail in linux - but I honestly don't care how it's set up, as long as it works and doesn't break, lol. Please help me make sure all my settings in linux are correct ... and keep in mind that I am NOT a linux guy.

Answer 1

The problem seems to be this part:
relay=10.20.51.30, dsn=5.1.2, stat=Host unknown (Name server: 10.20.51.30: host not found)
I assume that 10.20.51.30 is the ip address of your exchange server. Now, without seeing the actual configuration files, it's a bit hard to find out what you did wrong, but normally, when you specify a mail relay, you have to use either a) a real dns name, not an ip, or b) enclose the ip in [] brackets - just like the line before that one, which says relay=[127.0.0.1].
Try enclosing the 10.20.51.30 in [] brackets, or, use the name of the exchange server instead of the IP, and make sure your DNS server can resolve that name.